The invention relates to an objective lens for focusing a radiation beam to a first focus through a first transparent layer having a first thickness and to a second focus through a second transparent layer having a different second thickness. The invention also relates to an optical scanning device for scanning a first type of record carrier having a first information plane and a first transparent layer of a first thickness and for scanning a second type of record carrier having a second information plane and a second transparent layer of a second thickness different from the first thickness, comprising a radiation source for generating a radiation beam and an objective lens for converging the radiation beam through the first transparent layer to a first focus on the first information layer and for converging the radiation beam through the second transparent layer to a second focus on the second information layer. The scanning includes writing, reading and/or erasing information in the record carrier.
The transparent layer in optical record carriers has, in general, the function of protecting the information layer from environmental influences and providing mechanical support for the record carrier, i.e. it acts as a substrate for the information layer. The thickness of the transparent layer is a compromise between the desired stiffness of the record carrier and the numerical aperture of the radiation beam used for scanning the information layer. If for a new type of record carrier the numerical aperture is increased in order to increase the storage density of the information layer, it is often necessary to reduce the thickness of the transparent layer in order to reduce the influence of disc tilt on the quality of the radiation beam. As a consequence, there will be different types of record carrier on the market, having different thicknesses of the transparent layer. A compatible record player should be able to scan the different types of record carrier, irrespective of the thickness of the transparent layer.
The transparent layer, through which a radiation beam scans the information layer, introduces a so-called spherical aberration in the radiation beam. The spherical aberration is compensated in the objective lens, making the radiation beam near its focus substantially free from spherical aberration. If an objective lens compensated for a first thickness of the transparent layer is used for scanning a record carrier with a transparent layer of a second, different thickness, the quality of the focus will be deteriorated due to the under- or over-compensated spherical aberration.
The PCT application IB96/00182 to which Braat et al. U.S. Pat. No. 5,708,638 corresponds, describes a device for scanning optical record carriers of the first and second type. This device uses an objective lens designed for converging a radiation beam through the first transparent layer to a best focus on the first information layer. When scanning a record carrier of the second type, the objective lens forms a paraxial focus on the second information layer. The best focus of a beam is the point along the axis of the beam which has the highest intensity. The paraxial focus of a beam is the point along the axis of the beam through or towards which the paraxial rays of the beam are converged. The radiation reflected from the record carrier is detected by a radiation-sensitive detection system. When scanning a record carrier of the first type, the detection system uses all radiation in the reflected beam. When scanning a record carrier of the second type, the detection system detects only radiation from a central area of the cross section of the radiation beam. Since the objective lens is not designed for converging a radiation beam through the thickness of the second transparent layer, the radiation beam will incur uncorrected spherical aberration on passage through the second transparent layer. By restricting the detection to the central rays of the beam, the highly aberrated rays in the outer annular area of the beam will then have a reduced influence on the output signals of the detection system.
According to the invention the objective lens converges a radiation beam to a single best focus, the objective lens having an outer annular part which introduces a first spherical aberration in the radiation beam compensating for passage of the radiation beam through a first transparent layer having a first thickness, and a central part inside the annular part which introduces a second spherical aberration in the radiation beam compensating for passage of the radiation beam through a second transparent layer having a different, second thickness. In general, the annular part and the central part are concentric; the parts may be adjacent or be separated by an intermediate ring-shaped area.
When the objective lens converges a radiation beam through the second transparent layer, the rays of the beam passing through the central part form a second focus, the rays being corrected for the spherical aberration incurred in passing the second transparent layer. When the objective lens converges a radiation beam through the first transparent layer, the beam passing through the combined area of the annular part and the central part forms a first focus. In that case only the rays passing through the annular part are corrected for the spherical aberration incurred in passing the first transparent layer, whereas the rays passing through the central part are corrected for the spherical aberration incurred in passing the second transparent layer. The invention rests on the insight that the correction of the central part of the objective lens for a thickness of the transparent layer different from the thickness of the transparent layer for which the annular part is corrected has only a relatively small influence on the quality of the first focus.
European patent application nr. 0 610 055, to which Komna et al. U.S. Pat. No. 5,446,565 corresponds, discloses an objective lens combined with a hologram or grating. The lens is corrected over its entire area for the passage of radiation through a first transparent layer thickness towards a first focus. The grating diffracts part of the radiation in a central area of the incoming radiation beam into a sub-beam which, after refraction by the lens, is corrected for passage through a second transparent layer thickness towards a second focus. The objective lens and grating converges the incident radiation beam to an outgoing beam comprising two sub-beams having different vergences. Hence, the combination of the objective lens and the grating forms two best foci. In contrast, the objective lens according to the invention converges the radiation beam to a beam having a single vergence and forms a single best focus. When the objective lens according to the invention is of a refractive-only type, a reflective-only type or a refractive-reflective-only type, it passes substantially all energy of the incident radiation beam to the first focus, and does not branch off part of the energy to a sub-beam forming the second focus. The phrase `substantially all energy of the radiation beam` means that only normal losses due to reflections on transitions from one medium into another one, due to absorption within media, and due to opaque parts of the lens are disregarded.
A further aspect of the invention relates to an optical scanning device using an objective lens according to the invention. The scanning device is suitable for scanning a first type of record carrier having a first information plane and a first transparent layer of a first thickness and for scanning a second type of record carrier having a second information plane and a second transparent layer of a second thickness different from the first thickness. The scanning device comprises a radiation source for generating a radiation beam and an objective lens for converging the radiation beam through one of the transparent layers to a single best focus, and in which an outer annular part of the objective lens introduces a first spherical aberration compensating for passage of the radiation beam through the first transparent layer and a central part inside the annular part introduces a second spherical aberration compensating for passage of the radiation beam through the second transparent layer.
When scanning a record carrier of the second type, the part of the beam passing through the central part is well corrected for the thickness of the transparent layer of the record carrier. The improved correction of the central rays of the reflected beam results in an improved quality of the output signals of the detection system when scanning a record carrier of the second type. The invention rests on the insight that, when scanning a record carrier of the first type, the effect of the correction of the central part of the objective lens for a thickness of the transparent layer different from that of the layer through which is presently being scanned has a negligible effect on the output signals of the detection system.